A/B testing is a process used in marketing to isolate and test factors that affect performance of a marketing effort. For example, A/B testing is commonly used in direct mail marketing where different versions of a direct mail letter are sent to different groups of people. The response rates of people receiving the different letters are compared to determine things that worked and didn't work in the letters. Everything from the color of the ink to the appearance of the envelope can be tested using A/B testing.
A/B testing can also be used to isolate and test aspects of content on a website that impacts its effectiveness. For example, A/B testing can be used to determine whether one “place order” button is more effective than another at getting users to order a product.